1. Field of the Invention
The present invention relates to a lock-up clutch mechanism used for a torque converter of an automatic transmission of a vehicle, and more particularly, it relates to an improvement in a friction sliding surface of such a lock-up clutch mechanism.
2. Description of the Related Art
While a torque converter used in an automatic transmission provides smooth starting, acceleration and deceleration, it has a disadvantage that transmitting efficiency is worsened since a power is transmitted via fluid. Thus, it is designed so that, if a vehicle speed exceeds a predetermined value, an engine is directly connected to drive wheels by operating a lock-up clutch mechanism including a lock-up clutch, in order to reduce energy loss and also to reduce consumption of fuel.
In recent years, in order to further reduce the consumption of fuel, the lock-up clutch mechanism has been operated even when the vehicle speed is low. In this case, in order to reduce engine vibration and transmission shock during the low vehicle speed, so-called slip lock-up control in which lock-up control is performed while maintaining a slip amount at a predetermined number of revolutions has been performed.
Further, in automatic transmissions having torque converters, in order to further reduce the consumption of fuel by using the lock-up clutch from a lower vehicle speed range, generally, the lock-up clutch for the slip control is used.
Such a lock-up clutch is used so that, in the low vehicle speed range, a piston is urged against a friction surface of a front cover by a small hydraulic pressure difference to create a predetermined slip rotating condition between the piston and the front cover, and, whereas, in a high vehicle speed range, the piston and the front cover are integrally joined by a great hydraulic pressure difference, thereby achieving complete tightening.
For example, Japanese Patent Application Laid-open No. 11-230305 (1999) proposes a technique in which, in such a lock-up clutch, the friction surface is worked or machined so that, if the hydraulic pressure difference is great, the friction surface is urged with uniform face pressure.
However, in recent years, in order to reduce the consumption of fuel and to make the torque converter lighter and more compact, there has been requested to further reduce a diameter of the lock-up clutch; to this end, a lock-up clutch having a friction surface capable of providing a greater transmitting force during the complete tightening has been needed.
Thus, in the present invention, it is designed so that, for example, a cutting amount of an inner diameter side friction surface becomes greater than an amount through which an inner diameter side of a piston is deformed toward a front cover in such a manner that a maximum face pressure position of a friction surface during complete tightening between the lock-up piston and the front cover due to a greater hydraulic pressure difference is located toward an outer diameter side from the center in a radial direction, and more particularly, located at a half position in the outer diameter side. As a result, distribution of the face pressure along the radial direction forms a mountain shape with an apex situated at the maximum face pressure position.
As is in the prior arts, if the face pressure at a contact area is uniform, a sealing ability of the outer diameter portion is worsened and a tightening force is reduced due to reduction in an urging force caused by the escaping of the hydraulic pressure on the friction surface; however, in the present invention, since the face pressure at the outer diameter side is increased, the sealing ability at the outer diameter side is enhanced, and, since the center diameter of the face pressure is situated in the outer diameter portion, transmitting torque is increased, whereby it is possible to obtain a more compact lock-up clutch.
In general, the lock-up clutch can generate self-excited vibration called as judder, thereby worsening riding comfort considerably. The judder is greatly influenced, particularly by imbalance of circumferential face pressure distribution during the slipping on the friction surface. The imbalance of the face pressure distribution is greatly influenced by accuracy of the friction surface, so that the face pressure distribution is dispersed in the circumferential direction of the friction surface by small undulation of the piston of the lock-up clutch and/or distortion of an attachment bolt for a drive plate provided on the front cover.
In order to suppress generation of such judder and to stabilize the sealing ability, the friction surface at the outer diameter side and the friction surface at the inner diameter side are cut after they are bonded.
When the friction surface is used in the slip control, the friction surface is cut to follow the configuration of the front cover or the piston so that the uniform face pressure distributions can be obtained in the circumferential and radial directions of the friction surface.
Further, the friction material at the inner diameter portion acts as a support for the entire friction material and thus is important to ensure strength. Further, by preventing yield of the friction surface, plastic deformation can be prevented.